Electrodeionization (EDI) is known in the art as a process which removes ionized species from liquids, such as water, using electrically active media and an electric potential to influence ion transport. Examples of electrically active media comprise ion exchange materials and ion exchange membranes. In general “ion exchange materials” denotes solid (perhaps highly porous) materials that, when brought into contact with a liquid, cause ions in the liquid to be interchanged with ions in the exchange material. “Ion exchange membrane” or “ion selective membrane” generally denotes a membrane porous to some ions, perhaps containing ion exchange sites, and useful for controlling the flow of ions across the membrane, typically permitting the passage of some types of ions while blocking others. In general, ion exchange membranes selectively permit the transport of some types of ions and not others, and also block the passage of the bulk liquid carrying the ions. A combination of ion selective membranes and ion exchange materials are sandwiched between two electrodes (anode (+) and cathode (−)) under a direct current (DC) electric field to remove ions from the liquid. The electric field may be applied in a continuous manner or may be applied in an intermittent manner. Cation exchange materials (or cation materials for short) can be used to remove positively charged ions, such as calcium, magnesium, sodium, among others, replacing them with hydronium (H3O+ or H+) ions. Anion exchange materials (or anion materials for short) can be used to remove negatively charged ions, such as chloride, nitrate, silica, among others, replacing them with hydroxide ions. The hydronium and hydroxide ions may subsequently be united to form water molecules. Eventually, the ion exchange materials become saturated with contaminant ions and become less effective at treating the liquid. Once these materials are significantly contaminated, high-purity liquid flowing past them may acquire trace amounts of contaminant ions by “displacement effects.” In conventional deionization, the saturated (or exhausted) ion exchange media must be chemically recharged or regenerated periodically with a strong acid (for cation materials) or a strong base (for anion materials). The process of regenerating the ion exchange media with concentrated solutions of strong acids or strong bases presents considerable cost, time, safety, and waste disposal issues.
Continuous electrodeionization (CEDI), a subset of EDI, uses a combination of ion exchange materials and ion exchange membranes, and direct current in a manner so as to continuously deionize liquids and also to eliminate the need to chemically regenerate the ion exchange media. The “continuous” label of CEDI applies to the condition wherein the electric field may be applied to the apparatus in a continuous manner while product liquid is being produced. CEDI includes processes such as continuous deionization, filled cell electrodialysis, or electrodiaresis. The ionic transport properties of electrically active media are an important separation parameter.
In the EDI apparatus illustrated FIG. 1, contaminant ions migrate through the ion depletion chambers 103, 107 and into the electrode chambers 101, 109. The ion exchange material in the composite bed depletion chamber 105, anion depletion chamber 103 and cation depletion chamber 107 are regenerated by water splitting in the composite bed depletion chamber 105. Hydronium produced from water splitting migrates towards the cathode passing though the cation exchange membrane 106 of the composite bed depletion chamber 105, into the cation depletion chamber 107 and ultimately into the cathode chamber 109. Similarly, hydroxide produced from water splitting migrates towards the anode passing though the anion exchange membrane 104 of the composite bed depletion chamber 105, into the anion depletion chamber 103 and ultimately into the anode chamber 101. Electrochemically produced hydronium, which results from oxidation of water at the anode, maintains electroneutrality as hydroxide and contaminant anions migrate into the anode chamber. Similarly, electrochemically produced hydroxide, which results from the reduction of water at the cathode, maintains electroneutrality as hydronium and contaminant cations migrate into the cathode chamber. In the apparatus illustrated in FIG. 1, the feed water hardness must be less than about 1-2 parts-per-million (ppm) (as CaCO3), otherwise precipitation of calcium as calcium carbonate or magnesium as magnesium hydroxide may occur in the cathode chamber causing an increase in device resistance or an increase in the backpressure, decreased flow, and potential plugging in the apparatus. By flowing the electrode rinse first through the anode chamber and then through the cathode chamber, the hardness problem may be reduced since the anode electrode rinse is slightly acidic and thus will help minimize precipitation of calcium carbonate and magnesium hydroxide. Still, feed water with hardness above several ppm (as CaCO3) can cause problems in the apparatus. Another potential problem with this apparatus can occur in the anode chamber. Common anions such as chloride and nitrate can be oxidized in the anode chamber to form electrochemically active species (ClO2 and NO2, respectively). These electrochemically active species can damage the ion exchange material in the anode chamber resulting in decreased lifetime of the EDI apparatus.
Thus, there is a need for an EDI apparatus which reduces or overcomes problems arising from electrode fouling by precipitation or damage to the ion exchange materials of the electrode compartment by electrochemically active compounds (such as oxidizers) while maintaining some or all of the advantages of homogeneous-material ion depletion chambers.
FIG. 1 illustrates an EDI apparatus that may be used for “general purpose” liquid deionization. The apparatus comprises three ion depletion chambers, 103, 105, 107, and two electrode chambers, 101, 109, separated by four ion exchange membranes, 102, 104, 106, and 108. This configuration offers improved deionization capability but may add additional complexity or cost for applications where the deionization requirement is selective. For some applications, the required water purity may require the exhaustive removal of anions or cations, but not both. This is the case in many forms of chemical analysis where a specific element or ion or a group of elements or ions are of interest. For example, in ion chromatography, either anions or cations are typically analyzed using different chemistries. For anion analysis by ion chromatography, the water used to prepare eluent or dilute samples or standards should be free of all anions as any anion in the water will likely manifest itself and either affect calibration (non-zero intercept) or compromise detection by increasing background conductivity. Other examples requiring feed water sources free from specific ions are silicate analyzers, sodium analyzers or phosphate analyzers as typically used to monitor high purity water. In these applications, the primary requirement is that the feed water has concentrations of the analyte(s) at or near the lowest possible levels, typically sub-ppb (part-per-billion) or ppt (part-per-trillion). Since many of these analyzers are used on-line (continuous analysis), it is desirable to have a continuous, highly purified feed water source for the analyzer. Currently, there are no commercially available water purifiers which can easily interface with analytical instruments and supply feed water with extremely low contaminant levels of the analyte ions. Therefore, there is a need for a simple, cost-effective EDI apparatus that may be devoted to a specific purpose.